Automatic frequency control circuit



Sept. 16, 1952 L. D. SMULLIN 2,611,092

AUTOMATIC FREQUENCY CONTROL CIRCUIT Filed Jan. 3,1946

FIG. I a" T TUNABLE 'T MAGNETRON MAGNETRON g OUTPUT A CRYSTAL A5 1CAVITY TUNING ll" RESONATOR AMPL'F'ER MOTOR I H I i FIRST GAS r-WOBBULATOR TUBE 3- ALTERNATING P RGUIT GSUORURREGNET T SECOND GAS TUBECIRCUIT J T FIG.3 4|

4 TO TO ALTERNATING TUNNG ALTERNATING CURRENT souRoEus) CURRENT Z ToALTERNATING MOTOR SOURCE (l3) cuRRENT,souRoEus) l I I I 4 i W T0.

CRYSTAL (l4) G, 2 CAVITY RESONATOR u cou (2z) PERMANENT $33? MAGNETPLUNGER (21) R E Z B INVENTOR. LOUIS D. SMULLIN srssa xsaw ALTERNATING vcuRREN MW 254,11,

ATTORNEY Patented Sept. 16 1952 UNITED STATES PATENT OFFICE AUTOMATICFiiEQUENoYooNTRoL CIRCUIT Louis l Sinullin, Arlington, Mass, assignor,by mcsneassi'g'ninents, to the United States of Americaas represented bytheSecretary of War 'Agiiiitatisii'ianiiaiy 2, 1946, Serial No. 638,8856 (llaiina (Cl. 250 26i This invention relates'in' general to frequencycontrol system'for thermionic tubes having cavity resonators as resonantelements, and more particularly to systems wherein such control isautomatic.

' For some purposes in communication systems using ultra-highfrequencies, the transmitting tubes used are magnetrons employing cavityresonators as resonant elements. Magnetrons of this ty'pe'have theproperties of drifting in frequency as the tube is warming up, andhaving their frequency changed, or pulled, with a variation in loading.This frequency pulling is undesirable because it results in the receiverused inthe communication system being improperly tuned with respect tothe transmitter frequency a largeportion of the time. A system commonlyused to alleviate this diffic'ulty is one in which the receiver-ismonitored as to tuning during the war ceiver bandwidth is made wideenough to effectively'cover any frequency pulling that may occurafterwards. This system is undesirable in that the former procedure iscumbersome and the latter procedure is expensive in relation toequipm'ent' -and space requirements;

Among the objects of the present invention, therefore, are: I

1. To provide a magnetron capable of being tuned over a range offrequencies; and I 2. To provide a control system for automati callykeepingthe' frequency of the magnetron substantially constant.

In accordancetvith thepresent invention there is'jprovided amulti-cavity magnetron oscillator with a tuning means embodied therein.A portion of the magnetron output is fed intoareferencejcayityresonator, which is of such a size and shape as toberesonant at the desired magnetron operating frequency. The resonantfrequency of the cavity is periodicallyvaried back and forth aboutthisdesired frequency, The cavityoutput is rectifiedand fed into a gas tubecontrol circuit providing'an output of one polarity if the cavity istunedlb elowf thisdesired magnetron operating frequency and of theopposite polarity if the cavity istune above. ADJ-C. motor coupledlto'the gas tube-circuit varies the tuning means within themagnetron in sucha direction as to' keep' the magnetron operating at the resonantfrequency of the cavity,

Th" invention will best be understoodby reference' to the drawingsinwhich:

Fig.1 is a blockdia'gra'm of one embodiment of the present invention;

m-up pe'riod'oi" the magnetron, and the re- Fig. 2 isv a functionaldiagram of the Wobbulator cavity resonator arrangement used in thesystem of Fig. 1; and

Fig.3 is a circuit diagram of one arrangement for controlling the tuningof the magnetron.

Referring now to a description of the invention and to Fig. 1 there isshown a tunable magnetron In, a portion of the output of which iscoupled to a. cavity resonator II. It is assumed that the operatingfrequency of the magnetron is such that cavitylresonators must be usedas the res'o: nant elements thereof. Mechanically coupled .to cavity His, a wobbulatoi l2, whichis a device for transforming the alternatingelectric voltage from source [3 to an alternating periodic mechanicalmotion in a wall of cavity H. In this specification, a wobbulator willbe considered to be a device for performing the above operation.

The output of the cavity l I is rectified by crystal l4, amplified byamplifier l5 and fed to .a first and second gas tube circuit, !6 and i!re spectively, for control voltage forming purposes. Also coupled tovcontrol elements of the gas tubes in gas tubecircuits. l6 and I! arealternating voltages, .two each,from source !3. l.

The two gas, tubes, actingas. coincidence tubes,

take the several voltages fed thereto and. produce control voltagestherein which aremade to affect the duration of and length of time ofrotation of.

D.-C tuning motor l8. This .motor is coupled to the magnetron tuningmeans, which varies the. operating frequency of the magnetron. Thistuning meanslmay be a metallic slug, say, move able in and out of one ofthe magnetron cavities and so varying the resonantfrequency thereof.

Fig. 2 isra functional diagram. of .the.wobbu.- lator and cavityresonator arrangement. Cavity I l isshown with the main tuning plungerZlsituated in oneend thereof. This may be a cylindrical cavity operatingin the TE0,1,1 mode, in which case the movement of the plunger 2'! in ahorizontal plane may alter the resonant frequency of the'cavityappreciably An alternating voltage of transformer 3!. One half of thesecondary winding 32 of the transformer completes its circuit throughtetrode gas tube 33 and tuning motor [8, The other half of the secondarywinding 32 of the transformer completes its circuit through tetrode gastube 34 and tuning motor I 8. This center tap of this secondary windingis grounded. The connections to the tuning motor are such that currentthrough the circuit of gas tube 33 will cause the tuning motor to turnin one direction, and current through gas tube 34 circuit will give arotation in the other direction. 7 r

The rectified output of crystal I4 is coupled through condenser 35 tothe control grid of amplifier tube 36 and developed'across grid-leakresistor 31. The cathode of tube 36 is connected to ground throughresistor 38, and the plate of the tube is connected to one end of thesecondary winding 32 of the transformer through load resistor 39. Thesuppressor grid is connected to the cathode and the screen grid isreturned to one end of the secondary winding 32 through screen droppingresistor 40.

The plateof this amplifier tube 36 is coupled through condenser 4i tothe first grid of tetrode gas tube 33. The alternating voltage fromsource I3 is connected to the primary winding 43 of transformer 42, thesecondary winding 44 of which is connected through resistor 45 betweenfirst grid and cathode of tube 33. The second grid of gas tube 33 isconnected directly to the cathode.

The rectified output of the crystal I4 is also coupled through condenser46 to the control grid of amplifier tube 4'! and developed acrossgrid-leak resistor 58. The cathode of tube 4'! is connected to groundthrough resistor 48, and the plate of this tube is connected to theother end of the secondary winding 32 of the transformer through loadresistor 49. The suppressor grid is connected to the cathode and thescreen grid is returned to this same end of the secondary winding 32through screen droppin resistor 50. The plate of this amplifier tube 41is coupled through condenser to the first grid-of tetrode gas tube 34.The alternating voltage from source I3 is connected to the primarywinding 52 of transformer 53, the secondary winding 54 of which isconnected through resistor 55 between first grid and cathode of tube 34.The second grid of gas tube 34 is connected directly to the cathode.

Referring now to the operation of the system and to Fig. 1, the generaloperation will first be discussed. A portion of the output frommagnetron I i) is coupled into cavity resonator II. This cavity isquiescently tuned to the desired operating frequency of the magnetron,and the resonant frequency is periodically varied or wobbulated aboutthis desired center frequency by the wobbulator [2, which issynchronized by the voltage from the alternating current source I 3.

The cavity ll produces a large output-when its resonant frequency passesthrough the operating frequency of the magnetron. This output isrectified by crystal l4 and fed to amplifier l5. One of the gas tubecircuits, say circuit It, now conducts and couples the energy from theamplifier I 5 to the tuning motor l8. Gas tube circuits [6 and I! aresynchronized by the sine wave voltage from source l3 in such a mannerthat during the time the resonant frequency of the cavity H is below itsquiescent resonant frequency, gas tube circuit [6, say, is held out 01?by 4 the negative cycle of the sine wave impressed on its grid. When thecavity passes through the operating frequency of the magnetron,therefore. there will be a large output from cavity II and the outputfrom amplifier l5 will be coupled to the tuning motor through gas tube16. Similarly, when the wobbulator l2 causes the resonant frequency ofthe cavity H to be above the quiescent resonant frequency, gas tube l1,say, is held cut oif. In both cases the direction of rotation of thetuning motor is such that the tuning slug, say, which it controls movesso that the operating frequency of the magnetron is brought nearer tothe quiescent resonant frequency of the cavity ll. Any type ofmechanical tuning means may be used in conjunction with this system. thearrangement not being limited to the use of a tuning slug.

The wobbulator which is illustrated schematically in Fig. 2 operates asfollows. The cavity resonator ll, shown in the side view, may becylindrical in shape and operating in the TEo,1,1 mode. The main tuningplunger 2! is positioned so that the resonant frequency of the cavity isequal to the desired operating frequency of the magnetron. The whole ofthe opposite end of the cavity to that into which the tuning plunger isplaced may consist of a flexible diaphragm 23. Rigidly mechanicallycoupled to this diaphragm is a movable core 25, around which is woundcoil 22, through which passes alternating current from source l3. Thealternating magnetic field produced by the current in coil 22 reactswith the fixed magnetic field from permanent magnet 24. The result isthe oscillation, at the frequency of the alternating current from sourcel3, of the movable core 25 and so of the diaphragm 23.-

The resonant frequency of the cavity thus oscillates on a frequency axisabout the quiescent cavity resonant frequency. In one arrangement, thefrequency of the alternating current source was 60 cycles per second,and the diaphragm was constructed with a mechanical resonant frequencymuch greater than 60 cycles per second.

The circuit of Fig. 3 is one possible means to allow the output from thecrystal to be transferred into an electronic control voltage for thetuning motor. It is seen that the secondary winding of transformer 3.lmay be thought of as a push-pull source of power for two separatecircuits. One circuit embodies the upper half of the secondary winding32, the gas tube 33 and the tuning motor l8, being completed throughground. The other circuit consists of the lower half of the secondarywinding 32, the gas tube 34 and the tuning motor It, being completedthrough ground.

The alternating voltage on the control grid of the gas tube 33 is madeto be out of phase with the alternating voltage on the plate of thistube. This voltage is coupled to gas tube 34 in a similar out of phasemanner.

The operation of this particular part of the invention will now beexplained by means of an example. Let us suppose that during the timethe wobbulator causes the cavity resonant frequency to be below thequiescent cavity frequency the alternating voltage on the plate of tube.33 will be positive, and the voltage on the grid of tube 33 will benegative, with respect tothe cathode. The tube is thus held in a cut offcondition.

brought to the operating frequency of the ma netron, the cavityproducing a relatively strong output, which is rectified by the crystaland fed to both amplifier tubes 36 and 41. This energy cannot passthrough tube 41 because the plate is at a lower potential than thecathode, but the energy is amplified by tube 36 and coupled to the gridof gas tube 33. The grid voltage is now brought up to a suflicient valueto turn this tube on, thus allowing current to flow through a winding oftuning motor IS. The direction of current flow is so chosen that themotor turns in a direction so as to move a slug, for instance, so thatthe operating frequency of the magnetron is increased. If this frequencyhad originally been greater than the cavity quiescent frequency, thesignal from the crystal would have been amplified by tube 41, would havetriggered gas tube 34, and so would have allowed current to flow throughthe tuning motor in a different direc tion, say, so that the rotation ofthe motor would be opposite to that which was obtained in the previousexample.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the a pended claims. to cover all such changes andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. A cavity resonator-type, tunable magnetron oscillator operating at afirst resonant frequency, reference cavity resonator means having aquiescent resonant frequency, coupled to said magnetron and havin atleast a portion of one wall thereof of a flexible material, a coil woundabout a movable core which is mechanically coupled to the flexible wall,a source of constant magnetic field in the neighborhood of said coil andcapable of influencing the coil, a source of alternating current coupledto said coil, whereby the resonant frequency of said reference cavityresonator means is varied periodically, a rectifying means coupled tosaid reference cavity resonator, an electric motor, a firstmulti-element gas tube connected between said source of alternatingcurrent and said electric motor, a second multi-element gas tubeconnected between said source of alternating current and said electricmotor in an oppositely phased manner than the connection through saidfirst gas tube, means coupling said source of alternating current to acontrol element of said first gas tube in an opposite phase than thecoupling to the plate thereof, means coupling said source of alternatingcurrent to a control element of said second gas tube in an oppositephase than the coupling to the plate thereof, means coupling saidrectifying means to said control elements, and means mechanicallycoupling said electric motor to said tunable magnetron oscillator,whereby said first resonant frequency is held in the neighborhood of thequiescent resonant frequency of said reference cavity resonator.

2. An automatic frequency control circuit for stabilizing atunableoscillator at a desired frequency, including: tunable resonatormeans coupled to said oscillator, the resonant frequency of saidresonator means being adapted to be varied on both sides of saidfrequency by a source of alternating current; first and secondthermionic tube means coupled to the output of said resonator means inlike phase; adjustment means, adapted to be activated by either of saidthermionic tubes and coupled to said oscillator, for adjusting thefrequency thereof; and transformer means, adapted to be activated bysaid alternating current source and coupled to the anodes and grids ofsaid tube means to excite the anodes of said tube means in oppositephase, and to excite said grids in opposite phase with respect to theanodes of their respective tube means; whereby the output from saidresonator means will cause one of said tube means to conduct, activatesaid adjustment means, and thereby bring said tunable oscillator back tosaid desired frequency.

3. The circuit of claim 2, wherein said adjustment means comprisestuning motor means, the direction of rotation of said motor means beingdetermined by which of said first and second thermionic tube means isconducting at any given time.

4. The circuit of claim 2, further including rectifier means coupledbetween the output of said resonator means and the input to said firstand second thermionic tube means.

5. The circuit of claim 4, further including amplifier means comprisingthird and fourth thermionic tube means the outputs of which arerespectively coupled to said first and second thermionic tubes, theanodes of which are respectively coupled to said transformer means inthe same phase as those of said first and second thermionic tube means,and the grids of which are both coupled in like phase to the output ofsaid rectifier means.

6. A circuit for deriving control energy as a function of the differencebetween the resonant frequency of a tuned circuit and the frequency of acurrent exciting said circuit, comprising: periodic means forperiodically varying one of said frequencies; first and secondthermionic tube means, having their inputs coupled to the output of saidtuned circuit in like phase; means for exciting the anodes of both saidtube means in opposite phase by an alternating current equal to andsynchronized with the frequency of said periodic means; means forexciting the grids of both said tube means by said alternating current,in opposite phase with respect to the anodes of their respective tubemeans; and a circuit activated by said tube means for deriving saidcontrol energy.

LOUIS D. SMULLIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2.238.117 Koch Apr. 15, 19412,312,919 Litton Mar. 2, 1943 2,372,193 Fisk Mar. 22, 1945 2,384,380Isserstedt Sept. 4, 1945 2,393 284 Brown Jan. 22, 1946 2,404,568 DowJuly 23, 1946 2,406,856 Satterlee Sept. 3, 1946

